In a telephone system, trunk cables are those cables that connect large population centers one to another, e.g., city to city, and a central office to another central office. On the other hand, distribution cable is used in a section of the system which connects a central office to a particular subscriber. Three requirements of a fiber optic distribution cable stand out: (1) ease of fiber identification, (2) high fiber count, and (3) low attenuation loss by the fibers over a wide range, generally minus -40+ to 70.degree. C. at 1310 and 1550 nm wavelengths. Presently, a total fiber count of 192 per cable is considered a high count fiber optic cable. For purposes of this disclosure, a high fiber count cable is one that contains at least 300 fibers in the aggregate and more particularly between 300 to 420 fibers per cable. Generally, a high count fiber optic cable for the purpose of this disclosure is one of the "loose tube" variety in which a single tube may contain 300 or more fibers or a plurality of tubes contain in the aggregate a total of 300 to 420 fibers.
A workman in preparation of connecting a fiber in a distribution cable must obviously enter the cable. If such an optical fiber cable has 300 to 420 fibers, a workman has a problem of identifying a particular fiber without wasted time. In addition, the relative expansion/contraction of the structural elements comprising a high count fiber optic cable creates a problem of cable structural stabilization between the temperatures of -40.degree. and 70.degree. C. Failure to satisfactorily stabilize cable structure during contraction (cooling) of its component parts more often than not causes attenuation. See for example the teachings of U.S. Pat. No. 4,687,294, incorporated herein by reference.
A fiber optic cable of the loose tube variety, see U.S. Pat. No. 4,072,398 for example, has structural components made primarily from plastic and steel that, apart from the fibers themselves, may shrink as much as one percent (1%) when cooled from +70.degree. C. to -40.degree. C. At room temperature, fibers in prior art loose tube construction (contrary to that shown in U.S. Pat. No. 4,072,398) lie either loosely twisted or with no twisting in an uncoiled configuration generally along the neutral axis of the tube in which they are housed, generally not touching the tube interior. See FIG. 1 of this disclosure. As the tube and its fibers are cooled, the tube (generally made from plastic) shrinks causing the optical fibers in it to come in contact with the tube inner wall. Further cooling will cause the optical fibers to shape themselves into a coil like form, that more often than not comes in contact with the tube inner wall. See FIG. 2 of this disclosure. It is the bending (coil-like formation) like that of FIG. 2 that causes attenuation loss. Reversal of this process (warming the loose tube composite) relieves the bending and some of the attenuation loss brought about by cooling. It is the object of this invention to solve the problem of attenuation in high count fiber optic cable.